Very Rare Weapon (Dagger), Finesse, Light, Requires Attunement

• You gain +1 bonus to attack and damage rolls made with this magical weapon.

Slasher Smile. Once per turn when you hit a creature with an attack made with this weapon, you can force them to make a DC 15 Charisma saving throw. On a failure, an unnaturally wide malevolent smile appears on their face until the start of your next turn, 

• if the target doesn’t have a face, a similar smile appears on the most prominent or appropriate part of their body. 

Whilst affected by this feature, the target is forced to make an attack of opportunity against the first creature that provokes it. If the attack of opportunity hits, the provoking creature is affected as if it was hit by Grincleaver using the Slasher Smile feature, forcing them to make the Charisma saving throw, and making them suffer from the same smile as the initial target that lasts until the start of that initial’s target turn.

Killer Joy. When you kill a creature of size small or larger with this weapon, or as an action on such a creature within reach that is already dead, you can carve a smile on their face (or other prominent appropriate feature if necessary), marking the creature with an ominously orange glowing wound.
As a  bonus action, you can choose one such marked creature within 60 feet of you, and its dead body starts laughing uncontrollably, rises up and makes one melee attack against a creature of your choice within its reach. All creatures that are within 15 feet of the marked creature when you use this feature must also make a DC 15 Wisdom saving throw, taking 2d12 psychic damage on a failure, or half as much on a success.
The creature immediately falls down dead after it makes its attack. 

You can only have a number of creatures marked equal to your proficiency bonus, the creature that was marked the earliest loses its mark if you exceed this number.
When a creature loses its mark, the wounds on its face turn into a scar and stop emitting an orange glow.